1. Field of the Invention
The present invention relates to a power transfer mechanism for motor vehicles, more particularly to a power transfer mechanism adapted for use in combination with a power transmission of a four-wheel drive vehicle to transfer drive torque from the power transmission to a set of front-wheel drive axles and/or a set of rear-wheel drive axles.
2. Description of the Prior Art
Disclosed in Japanese Laid-open Utility Model Applications Nos. 61-200729 and 63-46231 is a power transfer mechanism of the type which includes an input shaft arranged to be applied with drive torque from a prime mover, a first output shaft arranged coaxially with the input shaft for drive connection to a set of rear-wheel drive axles, an intermediate hollow shaft in surrounding relationship with the input shaft, a second output shaft arranged in parallel with the hollow shaft for drive connection to a set of front-wheel drive axles, a planetary gearset mounted on the hollow shaft to transfer drive torque from the input shaft to the first and second output shafts, and a limited-slip differential arranged between the hollow shaft and the first output shaft to limit differential operation of the output shafts.
In such a power transfer mechanism as described above, the planetary gearset is of the single pinion type wherein the planet carrier is adapted as an input element in drive connection to the input shaft, the ring gear as a first output element in drive connection to the first output shaft, and the sun gear as a second output element in drive connection to the intermediate hollow shaft. The limited-slip differential is in the form of a viscous fluid coupling in drive connection to the first and second output elements of the planetary gearset. A drive train between the intermediate hollow shaft and the second output shaft is located at the front side of the planetary gearset and viscous fluid coupling.
In the above arrangement, the differential operation between the output shafts directly acts on the viscous fluid coupling, causing excessive wear of the components thereof. Assuming that the drive train between the intermediate hollow shaft and the second output shaft is located at the rear side of the limited-slip differential, the planet carrier is adapted as a first output element and the sun gear as a second output element. In such a case, the transfer ratio of drive torque to the rear-wheel drive axles through the first output element may not be increased since the ring gear acts as an input element.